V4L2VideoDevice: Call FrameBuffer::Private::cancel() in streamOff()

At the moment `V4L2VideoDevice::streamOff()` sets
`FrameBuffer::Private`'s metadata directly, while that's equivalent to
calling `FrameBuffer::Private::cancel()`. To ease code tracing, this
patch replace the manual modification with the function call.

Signed-off-by: Harvey Yang <chenghaoyang@chromium.org>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Umang Jain <uajain@igalia.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/v4l2_videodevice.cpp

@@ -2031,10 +2031,9 @@ int V4L2VideoDevice::streamOff()
 	/* Send back all queued buffers. */
 	for (auto it : queuedBuffers_) {
 		FrameBuffer *buffer = it.second;
-		FrameMetadata &metadata = buffer->_d()->metadata();
 
 		cache_->put(it.first);
-		metadata.status = FrameMetadata::FrameCancelled;
+		buffer->_d()->cancel();
 		bufferReady.emit(buffer);
 	}